1. Field of the Invention
The present invention relates to a power converting apparatus in which single-phase AC power is input and once converted into DC power and, thereafter, the DC power is further converted into three-phase AC power by using a DC/AC inverter, thereby driving an induction motor as a load.
2. Description of the Prior Art
As a power converting apparatus in which single-phase AC power is input and an AC induction motor is driven as in an electric vehicle, there has conventionally been known an apparatus of the type in which AC power is converted into DC power by using a PWM converter and is further converted into an AC having a variable voltage and a variable frequency by an inverter and is given to an induction motor. FIG. 1 shows an example of such a conventional power converting apparatus. Single-phase AC power is received by a pantagraph "PAN" and stepped down by a transformer "T" and input to a PWM converter "COV" through a reactor "L" and is once converted into DC power. A capacitor "CO" is connected to the output side of the converter "COV". Further, a three-phase DC/AC inverter "INV" is connected to the capacitor. The three-phase AC power is generated by this inverter "INV" and an induction motor "IM" is driven.
Such a type of power converting apparatus has been disclosed in, for instance, K. Nakamura et al., "Restraint Method of Beat Phenomenon on Converter-Inverter System", UDC, Vol. 109, No. 5, from pages 363 to 369, May, 1989.
However, such a conventional power converting apparatus has the following problems. That is, the PWM converter "COV" converts an AC to a DC, and the voltage and current on the DC side are smoothed by the capacitor CO. However, the DC voltage causes pulsatory vibrations unless the capacitance value of the capacitor CO is infinite. When the DC voltage including the pulsatory vibrations is used as an input power source and is converted into the desirable AC power by the DC/AC inverter "INV", a so-called beat phenomenon occurs and pulsatory vibrations are caused in the motor current due to the mutual interaction between the pulsating frequency and the frequency of the converted AC. Such a pulsatory vibration phenomenon not only gives pulsatory vibrations to the motor torque output but also causes a problem such that an instantaneous current from the DC/AC inverter "INV" increases.
Although the DC/AC inverter "INV" has the capability to control both of the output frequency and the output voltage, the output voltage can be controlled by controlling a conduction ratio of a switching element employed in the DC/AC inverter "INV", that is, by adjusting a pulse width.
There is, however, a problem that the peak value of the output voltage waveform of the inverter "INV" increases or decreases due to an influence by the pulsatory vibrations of the DC voltage, so that the voltage value fluctuates and the output current is also fluctuated.
In this case, since the peak value of the voltage cannot be controlled, the pulse width thereof is controlled as a practical control method. That is, when the peak value of the output voltage is high, the pulse width is controlled to be narrowed. When the peak value is low, the pulse width is controlled to be widened. In this manner, the average value or effective value can be controlled as a parameter of an area of the voltage. Eventually, the voltage fluctuation can be suppressed.
In the case of embodying such a conventional method, it is necessary to detect the pulsatory vibrations of the DC voltage and to control the output voltage waveform of the DC/AC inverter. However, since the capacitance value of the capacitor employed in the DC circuit is generally set as large as possible, the pulsatory vibrations themselves of the DC voltage is not relatively large, which is therefore difficult to be detected. It is preferable that the input AC voltage as a major factor of the pulsatory vibrations of the DC voltage is detected, and the fluctuation of the DC voltage is predicted on the basis of the detected input AC voltage, and thus, the predicted fluctuation is desirable to be used for the inverter voltage control.
The present invention has been made in an attempt to solve the problems in the conventional power converting apparatuses and therefore, has an object of the invention to provide a power converting apparatus which can reduce the pulsatory vibration phenomenon of a motor current and can avoid the instable operation of a DC/AC inverter in consideration of the above drawbacks.